1. Field of the invention
The present invention relates to a molding apparatus and a process for molding with a thermoplastic resin. More specifically, the invention relates to a molding apparatus and a molding process which can satisfactorily transfer a molding surface of a mold onto a molded product in a short cycle time.
2. Description of the Related Art
As a method for obtaining resin molded products, an injection molding process and a blow molding process are known.
The injection molding process is a system for injecting a molten resin into an enclosed mold at high pressure (200 to 1000 kg/cm.sup.2) to transfer the configuration of the molding surface of the mold onto the resin. Since this process employs high pressure, the configuration of the molding surface can be accurately transferred to the molded resin. Therefore, the injection molding process is suitable for molding a product having a highly smooth surface (hereinafter referred to as "mirror surface") or a grained surface. On the other hand, because of high pressure employed, the mold withstanding high pressure becomes necessary to make the construction of the mold complicated and to cause high cost. Therefore, the injection molding process is not suitable for a flexible manufacturing system. Also, since a special arrangement is required for forming a hollow product, it makes the production process complicated.
The blow molding process employs a system to supply a parison (hollow cylindrical resin in molten or softened condition) between mold halves, then to clamp the mold halves, and to feed a fluid into a hollow portion under pressure to urge the outer periphery of the parison onto the molding surface of the mold to transfer the configuration of the former. Since the parison is urged onto the molding surface by the fluid pressure, the pressure to be employed is relatively low (4 to 10 kg/cm.sup.2). The configuration of the molding surface cannot be transferred accurately onto the surface of the product. Therefore, the blow molding process is not suitable for production of the molded product having a mirror surface (highly smooth surface) or a grained surface. However, since the blow molding process is suitable for mass production of a hollow article, it is widely used.
Japanese Patent Application Laid-open No. 102734/1983 discloses a mold for blow molding having an inner thin wall mold and an outer cooling mold which can be contacted and released to and from the inner mold. In this mold, for the purpose of improvement of the surface brilliance of the blow molded product, the mold is preliminarily heated before supplying of a parison. After the parison is brought into contact with the molding surface of the inner mold, the inner surface of the outer cooling mold is contacted onto the outer periphery of the inner mold to quickly cool the inner mold to obtain a molded product.
In Japanese Patent Application Laid-open No. 77231/1992, it is disclosed that upon molding with contacting the parison onto the molding surface of a mold, a temperature of the mold is maintained within a range from around a temperature where crystallization speed of the parison becomes maximum to a molten point thereof for preventing die-line or weld-line from being remained on the surface of the molded product. In addition, by circulating a coolant in the hollow of the parison during molding, expansion of a cycle time of molding is prevented. Japanese Patent Publication No. 73903/1994 (corresponding to U.S. Pat. Nos. 5,017,126 and 5,190,715) discloses a mold for molding, in which a shell having a high thermal conductivity and having a large number of communication holes, is fixed on a vessel-shaped mold frame to form a cavity surface portion and an intermediate region at the backside thereof. In the intermediate region, resin or metal having a low thermal conductivity is filled, or, in the alternative, a reinforcing block having a communication hole is provided.
In the resin molding technology, there is a demand to obtain a resin molded article having a mirror surface or a grained surface employing a mold having a relatively simple construction. On the other hand, there is a demand to produce a hollow molding article (e.g., air spoiler for an automotive vehicle) having a mirror surface or a grained surface, in simple process.
In the mold for a hollow article disclosed in the above mentioned Japanese Patent Application Laid-open No. 102734/1983, the molding surface is accurately transferred by heating the inner mold. However, since the resin is cooled by relatively shifting the inner mold relative to the outer cooling mold, it is possible that the structure of the mold becomes complicated and to be weak, and the cooling period becomes longer. In this publication, there is no disclosure with respect to an optimal heating temperature and/or cooling temperature for making the surface of the molded resin article clear and for shortening an overall cycle time.
In the blow molding process disclosed in Japanese Patent Application Laid-Open No. 77231/1992 above, it is intended to make the molded surface clear by heating the mold and maintaining the temperature of the mold within a range from around a temperature where crystallization speed of the parison becomes maximum to a molten point thereof. In this process, however, since the mold is maintained at this temperature even during a cooling period, it is not so effective to shorten the cooling period. Moreover, the parison is cooled from inside by circulating a coolant in the hollow of the parison, making the temperature control complicate for maintaining the mold temperature at the above temperature.
The apparatus of Japanese Patent Publication No. 73903/1994 performs heating and cooling by passing a heating and cooling medium through a plurality of communication holes provided in inside and backside of the shell (mold surface portion) where the molding surface is formed, and by feeding a heating and cooling medium within the intermediate region at the backside of the molding surface. In the case of this apparatus, since heat transmission in the intermediate region is moderate, a cycle time can not be made shorter.